602 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



mainly dependent upon the character of blood supplied to it. Deficient 

 oxygenation of the blood acts as a stimulus to this centre and causes 

 the respiratory movements to become quicker as well as deeper, while 

 expiration becomes especially increased in power. In a greater degree the 

 deprivation of oxygen from the blood appears to cause the extension of 

 the stimulus from the respiratory centre to other adjoining motor centres, 

 and we then find that not only the ordinary muscles of respiration are 

 thrown into violent action, but that every muscle in the body which is 

 connected with respiration is thrown into forced contraction. Such a 

 state is described as dyspnoea. 



On the other hand, the blood may become overcharged with oxygen, 

 as by causing an animal to breathe an atmosphere overcharged with this 

 gas, or by making forced artificial respiration in one of the lower animals. 

 The blood then becomes saturated with oxyhemoglobin. The stimulus 

 of the respiratory centre is thus removed, and, as a consequence, we find 

 that in such an animal the motions of respiration cease, and the animal 

 may remain perfectly quiescent without breathing for a number of 

 minutes until the blood has freed itself from the excess of oxygen. 

 These facts show that a want of oxygen in the blood is a natural stimu- 

 lus to the respiratory centre. Blood which is poor in oxygen is also 

 to be regarded as rich in carbon dioxide. That it is the poverty of oxygen 

 and not the excess of carbon dioxide which brings this centre into 

 activity is shown by the fact that if an animal be caused to breathe an 

 atmosphere deprived of oxygen, or one which will not interfere with the 

 removal of carbon dioxide, the phenomena of dyspnoea will still appear. 

 Thus, an animal placed in an atmosphere of hydrogen will not be 

 prevented from removing carbon dioxide from its venous blood, but. will, 

 of course, be shut off from "all supply of fresh oxj^gen. In such an 

 animal the phenomena of dyspnoea rapidly appear. This action on the 

 respiratory centre is produced directly, and not through the mediation 

 of any nerves, for even when the spinal cord is cut and the supply of 

 oxygen from the medulla shut off the phenomena of dyspnoea are 

 evidenced in the spasmodic contractions of the nostrils and glottis, even 

 though their expression through the muscles of respiration is rendered 

 impossible. So,also,ligating the vertebral and carotid arteries produces 

 the same result by decreasing the supply of oxj-genated blood brought to 

 this centre. Again, if the blood distributed to the respiratory centre be 

 heated above normal, dyspnoea is also produced, from the fact that the 

 increased temperature of the blood leads to an increased activity of the 

 chemical processes in the body, and so to the more rapid exhaustion of 

 oxygen. Here, also, we find dyspnoea produced through the shutting off 

 of the supply of oxygen from this centre. 



As has been mentioned, if the supply of oxygen be interfered with, 



